1. Field of the Invention
The present invention relates to a method and apparatus for heating of air-tight packaged objects with microwaves and preferably under external excess pressure.
2. Brief Description of the Background of the Invention Including Prior Art
Methods and apparatus for using microwaves for heating purposes are known in various forms and shapes, where the microwaves are employed for heating purposes. For example, microwave provisions are employed for the thawing of deep-frozen foods. It is disadvantageous in this context that the objects can be provided only with a comparatively thin thickness, since these microwave provisions have only a small penetration depth, such that in case of a larger object cross-section no homogeneous warming is possible. This means in practical terms that objects with larger cross-section begin to thaw, but are not thawed completely or, respectively, cannot be heated within a short time up to their eating temperature because of the danger of causing damage such as the running away of the outer regions in the case cakes containing whipped cream or the charring of the outer layer in case of chicken.
In particular during heating above 80 degrees centigrade there arise problems for air-tight sealed objects, since the packaging is subjected to an increasing pressure with increasing temperature, which finally can result in a ripping open of the package. However, heating to above 80 degrees centigrade is required for sterilization purposes. Therefore, already microwave provisions have been constructed where the heating is performed under external pressure.
Thus packaged objects are sent through a transport tube, where the necessary overpressure is provided by the pressurized water according to U.S. Pat. No. 3,809,844. Here it is disadvantageous that the objects have to be very small for a homogeneous heating. Nevertheless, there are still problems with the homogenization in the edge and corner regions. One has attempted to balance these deficiencies based on additional steps and in particular by heating of the surrounding water. Thus it is required to achieve a temperature balancing via conventional heat conduction based on liquid or gaseous intermediate means in addition to the heating with microwaves.